Encyclopedia of Computer Science and Technology

file 191as dental and medical illumination and in experiments intransmitting images for television.DevelopmentOptical fiber in its modern form was developed in the1950s. The glass fiber through which the light passes issurrounded by a transparent cladding designed to providethe needed refractive index to keep the light confined. Thecladding in turn is surrounded by a resin buffer layer andoften an outer jacket and plastic cover. Fiber used for communicationis flexible, allowing it to bend if necessary.Early optical fiber could not be used for practical communicationbecause of progressive attenuation (weakening)of the light as it traveled. However, by the 1970s the attenuationwas being reduced to acceptable levels by removingimpurities from the fibers. Today the light signals can travelhundreds of miles without the need for repeaters or amplifiers.In the 1990s a new type of optical fiber (photonic crystal)using diffraction became available. This kind of fiberis particularly useful in applications that require higherpower signals.Communications and Network ApplicationsOptical fiber has several advantages over ordinary electriccable for communications and networking. The signals cantravel much farther without the need for a repeater to boostthe signal. Also, the ability to modulate wavelengths allowsoptical fiber to carry many separate channels, greatly increasingthe total data throughput. Optical fiber does not emit RF(radio frequency) energy, a source of “cross talk” (interference)in electrical cable. Fiber is also more secure than electricalcable because it is hard for an eavesdropper to tap.Today fiber is used for most long-distance phone linesand Internet connections. Many cable television systemsare upgrading from video cable to fiber because of itsgreater reliability and ability to carry more bandwidth andenhanced data services.The last area where electrical (copper) cable predominatesis in the “last mile” between main lines and housesor buildings, and within local networks. However, newbuildings and higher-end homes often include built-in fiber.Increasingly, phone companies are upgrading service bybridging the last mile through fiber-to-the-home (FTTH)networks. While requiring a considerable investment, FITHallows phone companies to replace relatively slow DSL withfaster (higher bandwidth) service better suited to delivervideo, data, and phone service simultaneously (see bandwidth,cable modem, and dsl). As of 2008, 3.3 millionAmerican homes had fiber connections, mainly throughVerizon’s FIOS service. It is expected that FTTH will bebuilt into many new housing developments.In 2007 Corning announced the development of “nanostructured”optical fibers that can be bent more sharply(such as around corners) without loss of signal. Corning isworking with Verizon to develop easier and cheaper waysto provide FTTH.(Related optical principles can also be applied to computerdesign. See optical computing.)Further ReadingThe Fiber Optic Association. “User’s Guide to Fiber Optic SystemDesign and Installation.” Available online. URL: http://www.thefoa.org/user/. Accessed September 20, 2007.“Fiber Optics: The Basics of Fiber Optic Cable: A Tutorial.” Availableonline. URL: http://www.arcelect.com/fibercable.htm.Accessed September 20, 2007.Fiber to the Home Council. Available online. URL: http://www.ftthcouncil.org/. Accessed September 20, 2007.Hecht, Jeff. Understanding Fiber Optics. 5th ed. Upper Saddle River,N.J.: Prentice Hall, 2005.Palais, Joseph C. Fiber Optic Communications. 5th ed. Upper SaddleRiver, N.J.: Prentice Hall, 2004.fileAt bottom, information in a computer is stored as a series ofbits, which can be grouped into larger units such as bytesor “words” that represent particular numbers or characters.In order to be stored and retrieved, a collection of suchbinary data must be given a name and certain attributesthat describe how the information can be accessed. Thisnamed entity is the file.Files and the Operating SystemFiles can be discussed at three levels, the physical layout,the operating system, and the application program. At thephysical level, a file is stored on a particular medium. (Seefloppy disk, hard disk, cd-rom, and tape drives.) Ondisk devices a file takes up a certain number of sectors,which are portions of concentric tracks. (On tape, files areusually stored as contiguous segments or “blocks” of data.)The file system is the facility of the operating system thatorganizes files (see operating system). For example, on DOSand older Windows PCs, there is a file allocation table (FAT)that consists of a linked list of clusters (each cluster consistsof a fixed number of sectors, varying with the overall size ofthe disk). When the operating system is asked to access a file,it can go through the table and find the clusters belonging tothat file, read the data and send it to the requesting application.Modern file systems further organize files into groupscalled folders or directories, which can be nested several layersdeep. Such a hierarchical file system makes it easier forusers to organize the dozens of applications and thousands offiles found on today’s PCs. For example, a folder called Bookmight have a subfolder for each chapter, which in turn containsfolders for text and illustrations relating to that chapter.Besides storing and retrieving files, the modern file systemsets characteristics or attributes for each file. Typicalattributes include write (the file can be changed), read (thefile can be accessed but not changed), and archive (whichdetermines whether the file needs to be included in the nextbackup). In multi-user operating systems such as UNIXthere are also attributes that indicate ownership (that is,who has certain rights with regard to the file). Thus a filemay be executable (run as a program) by anyone, but writeable(changeable) only by someone who has “superuser”status (see also data security).The current generation of file systems for PCs includesadditional features that promote efficiency and particularly